In residential and commercial buildings, most energy is spent to regulate the temperature inside the enclosure. Significant portion of energy is wasted by heat exchange through the envelope of enclosure. Conventionally, the envelope of enclosure is made of reinforced concrete. Albeit concrete is known as thermal insulating material, significant amount of heat is transferred through the wall, which is comparable to windows and open doors and it is not satisfactory to achieve the requirement of current green building standards on the thermal resistance of building envelope without extra enlargement over the structural and durability design requirements. It is possible, but not economical, to enlarge the element of building envelope with conventional concrete material to enhance the thermal resistance, which adds extra loads on foundations as well as load bearing elements. An alternative approach is to provide lightweight material with high thermal resistance to form the building envelope.
Expanded polystyrene (EPS) shows a good balance among the thermal resistance, strength and self weight. However, it is flammable and thereby fire can widely spread easily on its surface. In addition, toxic gas is released upon its thermal decomposition. Hence, it is prohibited to be used in buildings.
On the other hand, cement-based material is inorganic, durable and with excellent fire resistance. To enhance the thermal resistance of conventional cement-based material, the density is reduced by incorporating light-weight aggregates or air voids into the cement-based matrix. Lightweight aggregates include expanded clay, vermiculite, expanded perlite, hollow glass bead, etc, which can significantly reduce the density and hence increase the thermal resistance. For example, U.S. Pat. No. 5,641,584 disclosed a highly insulative cementitious matrixes, wherein lightweight aggregate is included to reduce the density and enhance the thermal insulation. However, the cost of lightweight cementitious composite made of lightweight aggregates is always significantly increased.
Another method to produce lightweight cement-based material is to incorporate air voids into the cementitious matrix and it is known as foamed concrete, aerated concrete or cellular concrete. There are mainly two groups of foaming methods for foamed concrete production. One is the chemical foaming method, while the other is the physical foaming method.
Regarding the chemical foaming method, chemicals, which can react in alkaline environment and release gas, are added to form an air void system inside cementitious matrix and reduce the density. Examples of such chemicals are aluminum powder, zinc powder, calcium carbide, hydrogen peroxide, magnesium peroxide and potassium manganite, etc. According to the curing way of foamed concrete after air void formation, the foamed concrete made with chemical foaming method could be classified to be autoclaved aerated concrete and non-autoclaved aerated concrete. Regarding the autoclaved aerated concrete, it normally contains 0.05-0.08 vol. % of aluminum powder which react with calcium hydroxide in cementitious matrix to form hydrogen gas. After hardening, the foamed concrete is cut and autoclaved at steamed environment of about 190 degree Celsius under pressure of 8-12 bars to achieve its full strength in a short term, saying 10-12 hours. The size of each member produced with this method is limited by the volume of the autoclaved chamber. While for the non-autoclaved aerated concrete, hydrogen peroxide which reacts under alkaline environment and releases oxygen gas is always added, and the foamed concrete produced are always cured under normal atmospheric pressure and room temperature. This method hence provides an economical way to produce lightweight cement-based material with high scalability. However, one problem of the chemical foaming method is that the rate of gas released highly depends on temperature and pH of the cement-based material in fresh state and hence, it is difficult to control the density of final product.
To overcome the high sensitivity to temperature and pH change when adding chemical to produce air voids, physical foaming method can be employed. The physical foam can be generated by mixing the foaming agent (surfactant), high pressure water and compressed air in a foam generation machine, and then mixed into the wet cementitious matrix. With this method, the density of foam generated can be measured and hence the density of the final product of lightweight cement-based material can be well controlled. The physical foam can also be induced by introducing gas directly or by introducing surfactant (synthetic) or protein foaming agent into the wet cementitious matrix, however, the density of foamed concrete made using this method is difficult to be controlled, as the volume of air voids formed highly depends on the rheology of wet cementitious matrix and the mixing way.
There are a lot of air voids, either open cell or closed cell, included and distributed in the foamed concrete, the thermal resistance and sound absorption capability of foamed concrete is hence high, but on the other hand, the moisture and/or water could hence be able to penetrate into foamed concrete easily, especially when the air void content is high and/or the open cell content is high. The thermal resistance and sound absorption capability of foamed concrete will degrade when water penetrates, accumulates and fills the air voids included in the foamed concrete. To reduce the ease of water absorption of foamed concrete, one approach is to make the cementitious matrix of foamed concrete, which is generally hydrophilic in nature, become hydrophobic, namely water repelling.
Shrinkage of concrete is the reduction in volume at constant temperature without external loading. And, it is an important material property that has significant effects on the long-term performance of designed structures, as serious shrinkage will lead to serious cracks in the concrete elements. Shrinkage can be classified into autogeneous shrinkage, drying shrinkage and carbonation shrinkage. Autogeneous shrinkage refers to volume changes caused by the hydration of cement. Drying shrinkage is resulted from the drying of cement and concrete materials. Carbonation shrinkage occurs when the hydration products of cement react with CO2 in the environment. As a result of the low content of aggregate, high content of reactive cementitious materials, and high content of air-voids which makes both drying and carbonation more easily, foamed concrete always shows a much higher shrinkage, saying 1-2 times, than the normal concrete. This large shrinkage will lead to serious cracks in foamed concrete elements, especially when reinforcements are embedded, which is harmful to both the insulation performance and durability of foamed concrete elements.
Lecomte, et al., U.S. Pat. No. 8,445,560 disclosed a granulated hydrophobing additive for rendering cementitious materials hydrophobic, wherein the hydrophobing additive comprises an organosilicon component and a binder polymer deposited on a particulate carrier so as to form a film. They also disclosed a cementitious material in powder form comprising dry cement and the granulated hydrophobing additive and a process for imparting to cementitious material a hydrophobic character. However, they did not provide the detailed composition of the cementitious materials and the detail of the process.
Shi, et al., US 2002/0117086 A1, disclosed a low shrinkage, high strength cellular lightweight concrete. However, a unique chemical admixture, shrinkage reducing agent, is required for such concrete to achieve low shrinkage.
It would hence be desirable to provide a lightweight cementitious matrix with both hydrophobic and low shrinkage characters, which have not been achieved by those presently available.